[Custom runtimes] Add readme for eoapi.dev deployment

[vincentsarago]

The eoapi.dev endpoints are deployed using the custom runtimes hosted in this repo. We use custom deployment code (should be similar to the one found in eoapi-template) and we should document (locally) how to update the stack

cc @emileten

[emileten]

@vincentsarago we have this already https://github.com/developmentseed/eoAPI/blob/main/docs/src/deployment.md#via-eoapi-cdk.

I think the only stuff it's missing is an explanation about the browser config.

Anything else ?

Or, should we rather add for internal development purposes a README in infrastructure/aws ?

[vincentsarago]

@emileten the deployment page within the docs should be for the general users. I'm talking more about the deployment code within this repo (which powers the eoapi.dev endpoints)